1. Technical Field
The present disclosure relates generally to refrigerant based heat exchange systems, and more particularly to vapor compression refrigeration systems including humidity control.
2. Description of the Related Art
Vapor compression type refrigeration systems are used for controlling temperature and humidity within conditioned spaces. In the residential context, such air conditioning systems may be used to cool the air of the living space to a temperature below the ambient temperature outside the residence. In industrial applications, refrigeration systems may be used to cool and condition the air within walk-in/drive-in coolers or freezers, such as for cooling and/or preservation of certain products.
Basic vapor compression refrigeration systems utilize a compressor, condenser, expansion valve and evaporator connected in serial fluid communication with one another forming an air conditioning or refrigeration circuit. A quantity of condensable refrigerant, such as R404 OR R517 refrigerant commonly used in refrigeration systems, is circulated through the system at varying temperatures and pressures, and is allowed to absorb heat at one stage of the system (e.g., within the cooled, conditioned space), and to dissipate the absorbed heat at another system stage (e.g., to the ambient air outside the cooled conditioned space). In the basic vapor compression refrigeration system, the evaporator is located within the conditioned space. Warm fluid, typically in the form of a liquid, is fed to the expansion valve, where the liquid is allowed to expand into a cold mixed liquid-vapor state. This cold fluid is then fed to an evaporator within the conditioned space.
The evaporator acts as a heat exchanger to effect thermal transfer between the cold refrigerant and the relatively warmer air inside the conditioned space, so that heat transfer to the refrigerant from the conditioned space vaporizes the remaining liquid to create a superheated vapor, i.e., a vapor that is measurably above its phase change temperature. This superheated vapor is then fed to a compressor that is typically located outside the conditioned space. The compressor compresses the refrigerant from a low-pressure superheated vapor state to a high pressure superheated vapor state thereby increasing the temperature, enthalpy and pressure of the refrigerant.
This hot vapor-state refrigerant is then passed into the condenser, which is located outside the conditioned space and typically surrounded by ambient air. Because the compressor sufficiently raises the pressure of the refrigerant, the resulting condensing temperature of the vapor is measurably higher than the ambient conditions surrounding the condenser. This temperature differential enables the condenser to effect a transfer of heat from the refrigerant to the ambient air as the high pressure refrigerant is passed through the condenser's heat exchanger at a substantially constant pressure.
This heat transfer effects another phase change in the refrigerant, from a hot vapor state to a slightly subcooled liquid state. This high temperature, liquid-phase refrigerant flows from the condenser and to the expansion valve to begin the process again.
As cold vapor-phase refrigerant passes through the evaporator as discussed above, the removal of heat from the conditioned air passing through the evaporator heat exchanger may cause the air passing through the heat exchanger to be cooled to below its saturation temperature, sometimes also referred to as a “dew point.” This cooling causes moisture to precipitate out of the conditioned air, typically causing liquid or frost to form on the fins of the heat exchanger. In liquid form the condensed moisture will drip down into a catch pan or conduit where the liquid water can be withdrawn from the conditioned space. In the frozen state, the moisture will remain on the evaporator surfaces until a defrost cycle is able to melt the frost and enable it to drain out of the pan. In this way, vapor compression type refrigeration systems are able to remove a certain amount of humidity from the conditioned space. The amount of humidity removed from the conditioned space is a function of the volume of air moved through the evaporator, the temperature differential between the refrigerant and the air as the air passes through the evaporator and the dew-point of the air. A greater temperature differential increases the amount of moisture that precipitates out of the conditioned air, thereby effecting greater dehumidification. Similarly, a greater volume of air moved through the evaporator and cooled to a given temperature can also effect greater dehumidification.
In some applications, it is desirable to maintain independent control over both temperature and humidity within the conditioned space. For example, in commercial refrigeration systems such as those used for food products, it may be desirable to maintain a particular set point temperature range while controlling humidity below a particular upper threshold, so as to maintain food products at their desired storage temperature while avoiding any collection of liquid or frozen water upon the food product or on interior cooler/freezer surfaces. Similarly, in some pharmaceutical applications, it is desirable to maintain specific temperature and humidity set point ranges in order to preserve the molecular structure, and thus the efficacy, of particular drug compounds.
In such applications, humidity control of the conditioned air must be effected without further cooling, and vice versa. Substantial design efforts have focused on variations of the standard vapor compression type refrigeration system to facilitate such independent control. For example, U.S. Pat. No. 6,826,921 teaches a “reheat” type heat exchanger within the conditioned space, and in a common air flow path with a traditional evaporator heat exchanger (see, e.g., heat exchangers 42 and 44 shown in FIG. 1). The reheat heat exchanger is selectively placed within the flow path of refrigerant by a separate and dedicated valve structure, and may be used to heat cold air coming from the output of the evaporator. This, in turn, allows the air passing through the evaporator to be made colder than it might otherwise be for a given temperature setpoint, such a larger amount of humidity precipitates from the air. Meanwhile, the reheat heat exchanger imparts additional heat to the outgoing air to avoid unwanted lowering of the set point temperature in the conditioned space while reducing the relative humidity of the air stream. However, both the evaporator heat exchanger and the reheat heat exchanger remain in the air flow of the fans moving air over the exchangers. This series type air flow arrangement through both heat exchangers permanently adds to the air flow restriction and requires additional fan power.